Low RPM cruise

[Exile]

Originally Posted by Wotname

I would say it was abnormal. Yes, you might get a temperature increase but not a dramatic one and not one where you have to back off after only a minute or two.

IMO, you have cooling issue, either a design issue or a defect.

Thanks. I'm inclined to agree and will pursue it further when I'm back onboard.

[bluenomads]

It's interesting what Perkins says for our engines:
Rating definitions

• Pleasure duty: For vessels operating up to 30% load factor. This rating is intended for pleasure/non-revenue generating applications that operate less than 500 hours a year. Typical applications could include but are not limited to: High speed planning craft.
• Light duty: For vessels operating up to 50% load factor. This rating is intended for applications that operate less than 1500 hours a year in variable load applications where full power is limited to 2 hours out of every 12 hours of operation and reduced power must be at or below 200 rpm of the maximum rated rpm. Typical applications could include but are not limited to: planing / semi displacement craft such as customs and police launches, sport fish charter vessels, passenger carriers, survey craft and long distance cruisers etc.
• Medium duty: For vessels operating up to 60% load factor. This rating is intended for applications that operate less than 4000 hours a year. Typical applications could include but are not limited to: Semi-displacement / displacement craft such as customs and police launches, high speed commercial fishing, passenger carriers, survey craft, ferries and long distance cruisers etc.
  
• Heavy duty: For vessels operating up to 80% load factor. This rating is intended for applications that operate less than 4000 hours a year. Typical applications could include but are not limited to: semi-displacement / displacement craft such as customs and police launches, high speed commercial fishing, passenger carriers, survey craft and ferries etc.

So it seems that 2 hours out of 12 of "full power" is fine (and that's light duty!).
"reduced power" (ie. not full) is 200rpm under maximum, or 2400rpm for these motors.

[hellosailor]

"Well that has never made sense to me, especially the place it into neutral part."
I'd guess that since this is done hourly, the carbon deposits have not yet had a chance to really bond onto the engine parts. So putting the engine in neutral allows you to reach higher speeds (no loads to slow things down) perhaps just higher speeds in the turbine gas path, and those higher speeds allow the turbine to "sling off" the carbon deposits before they have a chance to become bonded.
Perhaps they just say "neutral" because that gives them a relatively known starting point, i.e. no load on the engine means that "half power" will always be reaching specific speeds. So that "neutral" per se isn't important, but it is simpler than trying to tell folks different rpm's for different installations, prop and hull fouling, transmission rations, etc.

[a64pilot]

Originally Posted by Exile

For better or worse I do not do this regularly with my non-turbo diesel, but the few times I have (mostly when testing rpm with a newly pitched prop) the coolant temp has shot up quite dramatically, i.e. to the point where I feel I need to back down after only a minute or two. Under normal running the coolant temp stays where it should. I've confirmed gauge accuracy with a laser heat gun, and have done the usual checks on both sides of the cooling system.



Anyone else have this experience? Is this normal?

No,
That is the acid test to tell if your cooling system is healthy or not.
A healthy cooling system, you cannot overheat, there is a designed in excess cooling capacity.
This is an excellent reason to occasionally do WOT ins with a fully warmed up engine, your engine should not exceed the fully open thermostat temp

[Delfin]

Originally Posted by sailorboy1

Yes if I also run at lower rpm and at say 5 knots I will use less gallons/hour. But, far aas miles/ gallon it makes little difference if I run at 5 knots or 6.5 knot (measured at calm conditions). So it’s worth knowing that if you have to motor then doing so at 6 knots doesn’t cost anything really (it does but s so little I can’t meadure it), but gets you there sooner.

Not looking to argue or debate as that’s what over 2 years of tracking tells me.

Not sure what the difference is, but with 2450 gallons of fuel, my range is around 4500 miles at 8 knots and 7000 miles at 6.8 knots. Same with airplanes. Long distance cruise is at a slower speed, which you use if you want to get the maximum distance out of a given amount of fuel.

[a64pilot]

Originally Posted by hellosailor

"Well that has never made sense to me, especially the place it into neutral part."

I'd guess that since this is done hourly, the carbon deposits have not yet had a chance to really bond onto the engine parts. So putting the engine in neutral allows you to reach higher speeds (no loads to slow things down) perhaps just higher speeds in the turbine gas path, and those higher speeds allow the turbine to "sling off" the carbon deposits before they have a chance to become bonded.

Perhaps they just say "neutral" because that gives them a relatively known starting point, i.e. no load on the engine means that "half power" will always be reaching specific speeds. So that "neutral" per se isn't important, but it is simpler than trying to tell folks different rpm's for different installations, prop and hull fouling, transmission rations, etc.

Drive or operate any diesel with a boost gauge, you don’t get much boost at all in neutral if you hold one wide open, but load one down where the fuel pump is really pouring the fuel in, and you get boost, lots of boost, and boost is turbine RPM of course. A turbo is driven by heat, heat that expands air of course, but it recycles heat.

It’s important to remember that our throttles are not throttles like we have in gas motors, it’s actually the Governor your controlling, half throttle is half RPM and only enough fuel will be added to achieve that RPM.
If you have ever run a farm tractor, it’s real apparent when you drop the plow, RPM stays the same, but there is exhaust smoke and the turbo starts whistling, you can really hear the engine start working, but RPM is constant.
Point is that full governed RPM in neutral is very little fuel, not all that much heat, your only burning enough fuel to spin the engine up, not doing any work.

Just racing an engine is not a real effective way to “blow it out” but if your at anchor charging batteries, what other method do you have?
Yes, some of us that are smarter than the average bear will run it in reverse, cause we know our boats and know not to over stress a cleat etc.
But an engine manufacturer can’t recommend it, cause if they did some idiot would tear out a cleat, do tremendous damage, and sue the engine manufacturer, cause the boat broke, and the engine manufacturer has no control over the boat.

[Simi 60]

From someone who probably knows more about diesel engines than anyone commenting

.
Revisiting the “Detroit” mystique again, its longevity was built on engines rated to run at 1900-2100 RPM and above, but could only last for 30+ yrs when operated continuously at 1100-1600 RPM (again, well under 50% of rated HP)..These same engines in a “crew” boat used in the off-shore oil industry, would go through “top-ends” (or worse) just about yearly when run at close to their governor settings.

Snipped

.The longest-lived engines that I’ve been involved with (hrs and yrs wise), have been engines in commercial or recreational trawler type applications run at 50% of rated HP or less..Yes, there are many other parts of the equation that leads to the life of a diesel engine, but I know from experience that running them slow (i.e. cruising for days on end at hull speeds or less) is NOT a cause for concern.

Snipped

In closing, I’ll mention that although this topic is brought up quite often and many people preach that you’ve got to use a diesel hard if you want it to last, I’m still waiting to find one that was rebuilt before its time due to low speed use..Just the opposite seems to be always the norm

https://www.sbmar.com/articles/low-s...arine-diesels/

We run our 855 Cummins around hull speed anywhere between 1000 and 1200 rpm for 7 to 8 knots for days on end.
I have been told by those that actually know, that as long as the engine and oil reach temp, all is good.
I have an inexpensive probe on the oil filter and it always gets around 70c plus even running at 1000 rpm.

[bluenomads]

Originally Posted by a64pilot

Point is that full governed RPM in neutral is very little fuel, not all that much heat, your only burning enough fuel to spin the engine up, not doing any work.

Hmm, I hear what you say over tractors.
But I've understood from people measuring fuel flow with meters that they saw very little difference in consumption in neutral compared to quite low rpms but at load.
I'm making up exact numbers as I can't find the references right now, but it was like if consumption at 1300rpm at load was a "100", then idle/neutral at 850rpm was an 80+ in terms of amount of fuel consumed. So just spinning up and running a big diesel unloaded used a lot of fuel vs loaded at low rpm (higher rpm and load of course increased fuel flow a lot).
This seems to be supported by the engine fuel curves which flatten out at low rpms, meaning not much difference.

Have you found this?

[bluenomads]

Originally Posted by Simi 60

I have an inexpensive probe on the oil filter and it always gets around 70c plus even running at 1000 rpm.

Hey Simi, what probe do you use?

[Simi 60]

Originally Posted by bluenomads

Hey Simi, what probe do you use?

I have this $10 one on the exhaust elbow of the genset.
Water flow slows and at 47c it goes off

https://m.ebay.com.au/itm/DC12V-24V-...vXJCeSWujZMGtg

Instructions here
http://www.autospeed.com/cms/article...splay&A=112617

I have this $130 one on the primary doing the same + one on the oil filter

https://m.ebay.com.au/itm/LOW-COOLAN...kAAOSwkLhaCjZa

[Exile]

Originally Posted by a64pilot

No,
That is the acid test to tell if your cooling system is healthy or not.
A healthy cooling system, you cannot overheat, there is a designed in excess cooling capacity.
This is an excellent reason to occasionally do WOT ins with a fully warmed up engine, your engine should not exceed the fully open thermostat temp

Makes a lot of sense a64. By "fully open thermostat temp" do you mean the 180ºF that my particular boat engine is set at? (200ºF for my truck). My engine temp is closer to 190ºF when I'm running above about 60% or ~2100 rpm. Ditto on my truck (maybe 210ºF) if I'm pulling a heavy trailer up a grade.

I don't want to further sidetrack the thread but just wanted to be sure what you meant by fully open thermostat temp.

Thanks.

[oldtom]

Originally Posted by StuM

Strange, everyone that I've seen giving actual data gets much better mpg at lower speed.



In my case it makes a big difference. Running on one engine:
5 knots - 2 lph = 2.5 nm /l = 9.5 nmpg
6 knots - 3.3 lph = 1.8 nm /l = 6.8 nmpg

6.5 knots - 5 lph = 1.3 nm /l = 4.9 nmpg


i.e Almost double the range or half the cost per mile at the slower speed.

To get the best out of any engine you have too no where it get's max torque in its R.P.M range any there for = less fuel burn to distance travel ,the speed will depend on speed of out put Rpm to prop with idyll of R.P.M (800) that in the same boat you get better fuel consumption with a in board engine /motor than a out board Hi RPM Prop Speed. you can cruse at the max torque setting all day&night and have no affect on a engine look at diesel genset large or waterpump set to run at there max engine torque load setting just have temp, lube, watched,have ran diesel 24x7doing both shut down for oil change and run up at idle check leak's and put back on full load

[markpierce]

It depends on the particular engine and boat.

Have a JD, NA 80-HP engine and a 14-ton boat. Don't like to operate less than optimum operating engine heat. On mine, 1400 RPM provides maximum torque generating sufficient heat, using 1.2 gallons an hour with a 30% load for 5.7 knots. Regardless, usually operate at 1800 RPM (43% load) for 6.3 knots and 1.7 GPH.

[Delfin]

Originally Posted by oldtom

To get the best out of any engine you have too no where it get's max torque in its R.P.M range any there for = less fuel burn to distance travel ,the speed will depend on speed of out put Rpm to prop with idyll of R.P.M (800) that in the same boat you get better fuel consumption with a in board engine /motor than a out board Hi RPM Prop Speed. you can cruse at the max torque setting all day&night and have no affect on a engine look at diesel genset large or waterpump set to run at there max engine torque load setting just have temp, lube, watched,have ran diesel 24x7doing both shut down for oil change and run up at idle check leak's and put back on full load

Since diesels have essentially flat torque curves above a fairly low rpm, and since the torque required to move a boat through the water at less than hull speed is minimal, I think there is a bit more to it than that. As the speed through the water increases, torque demand increases until max torque is reached, and fuel consumption increases accordingly. Below that, the boat can move through the water at 85% of hull speed and you'll never see max torque, fuel efficiency in terms of range increases and wear on the engine decreases.

[Dockhead]

Originally Posted by bluenomads

Hmm, I hear what you say over tractors.
But I've understood from people measuring fuel flow with meters that they saw very little difference in consumption in neutral compared to quite low rpms but at load.
I'm making up exact numbers as I can't find the references right now, but it was like if consumption at 1300rpm at load was a "100", then idle/neutral at 850rpm was an 80+ in terms of amount of fuel consumed. So just spinning up and running a big diesel unloaded used a lot of fuel vs loaded at low rpm (higher rpm and load of course increased fuel flow a lot).
This seems to be supported by the engine fuel curves which flatten out at low rpms, meaning not much difference.

Have you found this?

You're saying something different, I believe, from what A64 was talking about, but it's interesting.


At idle, our diesels do have a load -- they are spinning all the rotating parts, turning the alternator and the pumps. That load is what makes up the "overhead" basic fuel consumption. It might be enough in some cases to create a reasonably warm combustion chamber -- at idle. Increase the revs, though, and all bets are off, because the same load which means the same amount of fuel burned will be done with greater flow of air through the engine.